The invention relates to a device for determining the position of an axially movable body and, in particular, to a device for determining the position of an axially movable body wherein the position of the body is deduced from the inductance of a coil arrangement. A layer whose magnetic properties modify the inductance of the coil arrangement is deposited on the body. An axial movement of the body, to which the layer is joined, modifies the magnetic flux in a coil arrangement comprising at least one coil and the change in the inductance resulting therefrom is measured and the position of the body is deduced therefrom.
In a device of the generic type, for example German Patent Document DE 40 20 369 A1, a ferromagnetic layer is deposited on a body whose position is to be determined. A layer composed of a material which has good electrical conductivity but is not ferromagnetic is additionally deposited on said ferromagnetic layer in order to cope with the problem of the eddy currents which occur. The ferromagnetic layer deposited on the body modifies the inductance of a coil in the event of movement. The position of the body is then determined by evaluating the inductance.
It could be regarded as disadvantageous in this connection that a second layer has to be deposited in order to cope with the problem of eddy currents because one more operation has to be expended on the deposition. In addition, problems may occur in the finished product because of, for example, mechanical stresses occurring as a consequence of different thermal coefficients of expansion in the two layers. Furthermore, the thicknesses of the layer and the frequency of the alternating current flowing through the coil have to be matched to one another, thereby imposing limitations on the flexibility of use of the device.
To determine the position of an axially movable body, German Patent Document DE 37 12 054 A1 discloses the possibility of producing the body from a soft-magnetic material, then introducing the body into a coil arrangement and thus determining the position from the change in the inductance.
If the position of an axially movable body which cannot be produced from a soft-magnetic material for reasons of, for example, the mechanical material hardness required (for example ceramic) is to be determined, the subject described in German Patent Document DE 37 12 054 A1 cannot be used.
Furthermore, a device is known in which a permanent magnet is moved in the interior of a coil. The velocity of the permanent magnet can then be determined from the induced voltage. In principle, it is consequently possible to obtain the position of the permanent magnet by integrating the velocity. Such an arrangement is referred to as a plunger coil arrangement.
It could be regarded as disadvantageous in this connection that errors may occur in the integration. Such an arrangement is also only of limited usability since it is restricted to the use of permanent magnets.
There is therefore needed a device for determining the position of an axially movable body in such a manner that the device has as high a response sensitivity as possible and, at the same time, operates as independently as possible of ambient conditions and has various possible application possibilities.
These needs are met according to the present invention, in a device for determining the position of an axially movable body wherein the position of the body is deduced from the inductance of a coil arrangement. A layer whose magnetic properties modify the inductance of the coil arrangement is deposited on the body. An axial movement of the body, to which the layer is joined, modifies the magnetic flux in a coil arrangement comprising at least one coil and the change in the inductance resulting therefrom is measured and the position of the body is deduced therefrom. The layer deposited on the body by atomic growth has a layer structure containing less than 8% by weight of phosphorus, preferably up to 3% by weight of phosphorus, up to 2% by weight of an element of main group IV or V, in particular antimony or lead, and up to 5% by weight of a transition metal element, in particular cobalt or iron. The percentages by weight resulting from the sum are increased up to 100% with nickel.
In the subject of the present invention, a soft magnetic layer is deposited in the form of a ring on the axially movable body on one part of the outer circumference. The soft-magnetic layer interacts with a coil arrangement comprising at least one coil. As the position of the soft-magnetic layer changes as a result of an axial movement of the movable body, the inductance of the coils or the coil changes. The inductance is consequently a direct measure of the position of the axially movable body.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.